1. Field of the Invention
This invention relates to a new improved ion and plasma source antenna. Ion sources are used in ion beam and neutral beam accelerators, spectrometers, for ion implantation, waste control of radioactive nuclear materials, and in plasma processing. Plasma processing encompasses the use of plasmas for surface treatment or surface modification including but not limited to ion implantation or coating of surfaces. Ion beams may also be useful for fusion. The positive or negative ions are generally obtained or extracted from a plasma that is created from a low pressure gas in a vacuum chamber. The gas is ionized by electron bombardment, vacuum arc discharge, thermal filaments, or power coupled from a power source to the gas via an antenna. This invention relates to a new improved radio frequency driven antenna used to create a plasma from which ions are extracted.
2. The Prior Art
Generally an ion source comprises a vacuum chamber into which a gas is introduced. The gas is ionized into a plasma through electron bombardment, vacuum arc discharge, or microwave or radio frequency (RF) power that is coupled to the gas volume by an antenna [The Physics and Technology of Ion Sources, Ian G. Brown, ed.; John Wiley & Sons (NY: 1989)].
One common method of producing a high density plasma in an ion source is to provide thermionic cathode filaments which emit a copious supply of electrons, which then may be accelerated to produce an ionized gas plasma. This approach has the disadvantage that thermionic cathode filaments often have a very short operating life of only a few hours, for example. Moreover, the electrically heated filaments produce considerable radiative heat which may cause operating problems such as material evaporating from the filament, or material being sputtered from the filament. The filament material that boils off or is sputtered off is a unwanted source of impurities in the plasma.
Another method of producing a dense ionized gas plasma is to supply RF power to the vacuum space. Sometimes a small thermionic cathode filament or other type of ionization starter is provided to emit electrons so that there is initial ionization of the ionizable gas. The RF power then provides additional energy so that a dense ionized gas plasma is produced. The RF power can be thought of as heating or increasing the energy level of the ionized gas so that a dense plasma is produced.
The power is supplied to the ion source by an antenna in the vacuum chamber. The antenna coil has power lead-ins which extend through seals in the walls of the vacuum chamber and are connected to an amplifier or an oscillator, outside the vacuum chamber. The power frequency is in the range of one to hundreds of megahertz.
The antenna often takes the form of an elongated electrical conductor formed into a coil.
Frequently, the antenna coil may be made of copper tubing.
Problems have been encountered with such antenna coils. When the antenna coil is made of bare metal, such as copper, sparking or arcing may occur in the vacuum chamber, both between the turns of the coil, and also between the coil and various electrodes which may be employed in the ion source. When the antenna coil is operated at high power levels, the RF voltage between different portions of the coil may be quite high. Moreover, electrodes may be employed in the ion source to produce accelerating voltages which are quite high, so that sparking or arcing may occur.
When a bare antenna coil is employed in an ion source, problems are often encountered with sputtering of the copper or other metal from the antenna coil, due to ion bombardment of the antenna coil. The sputtered copper or other metal is deposited on other surfaces within the vacuum chamber of the ion source and may cause problems such as current leakage or short circuits between electrodes.
An attempt has been made to deal with these problems of voltage breakdown, sparking, arcing and sputtering by covering the bare antenna coil with sleeving material made of woven glass, quartz fibers, or ceramic to act as electrical insulation. This approach reduces sputtering but does not eliminate sputtering as a problem. Moreover, the woven glass or quartz sleeving provides only limited protection against voltage breakdown, sparking and arcing, without eliminating them as problems. With the ceramic tubing, the electric field is attenuated too much for good power application to the gas volume via the antenna.
Sleeving does not make good thermal contact with antenna which results in problems with overheating. Moreover, the woven glass or quartz sheathing introduces the additional problem of causing the evolution of contaminating gases, such as oxygen and water vapor, which are driven out of the woven glass or quartz material during the operation of the ion source, largely due to the heat generated in the ion source during normal operation.
In 1988 researchers developed a glass-coated copper-coil antenna on which the glass coating was fused directly on to a copper antenna (U.S. Pat. No. 4,725,449, Ehlers & Leung). It had several advantages over previous RF coupled antennas. Glass is electrically insulating and allows the exterior of the antenna to float at a lower negative potential than that applied to the antenna. This reduced the sputtering of antenna material by the surrounding plasma ions. The glass coating is flexible enough that it doesn't crack from vibration during operation and the coefficient of thermal expansion of the glass coating is sufficiently close to copper's to resist some thermal stress. However, the glass coating is very fragile, frequently becomes porous, and is not as good an electrical insulator as would be desirable. It's performance deteriorates in the presence of corrosive gasses. It is limited to a power of 25 kilowatts. High energy electrons appear to pierce the glass wall and destroy the electrically insulating quality of the coating. This happens more quickly at high power operation than at low power operation. It also exhibits a limited lifetime at lower power operation because there is always some distribution of particles present with high energy, even in a low energy plasma. Practically, the glass coating is not an effective electrical insulator at powers above about 25-30 kW.
It would be extremely desirable if an antenna could be constructed with a coating material that had greater mechanical strength and higher dielectric constant than those currently available while maintaining good thermal contact with the exterior of the antenna. It would be even more desirable to construct an antenna capable of coupling more than about 25 kilowatts power to the plasma. It would be yet more desirable to have an antenna that would last longer than the fragile glass-coated antenna. It would be most desirable if an antenna was available that resisted the corrosive effects of many plasma gasses, for example, BF.sub.3. It would be even more desirable to be able to produce an antenna coated with a thin layer of very dense, hard, mechanically stable, slightly flexible, nonconducting material that fused directly to the antenna metal.